I have been having a strange problem and am not really sure where to start diagnosing.

After the car is up to operating temp, whenever I shift to neutral, if the RPM's are above 2k, the car idles at about 1800rpm until I start again. If I keep the car in gear and wait until the RPM's are below ~1500, then it drops down to about 800rpm, and idles as normal.

Everything else seems to be fine with the car's mechanical, just this one weird glitch....

Is it the decel valve? Something else?

Typical symptom of a bad decel valve. Will maintain high idle. From a stop, if you hold the brake, and let out the clutch slightly to drag the engine speed down, then it will stay down.

A way to test and be sure is to simply disconnect and plug all 3 lines going to the decel valve. Car should operate normally with the exception that the engine will drop RPM's faster than with the decel valve connected.

Demick

I don't doubt you Demick, but if that were true, then why would the cars need the decel valve in the first place?

Mostly for emissions.

Because you don't want to drop to idle rpm after you get off the thottle and have too much engine braking.

If it turns out to not be the decel valve...

Check out the advance plates and the vacuum canister on the distributor. My car was doing this at one point, and the plates were gunky. Refurbed the dizzy, clean, lube, and replaced the vacuum unit (which was leaky) and it cured my problem. (Don't know which year/motor you have - my 75 2.0 used vacuum retard at idle on the dizzy, yours may not.) But the dizzy might need some maintenance anyway.